Conventionally, as reflective electric address spatial light modulators, an LCOS-spatial light modulator (LCOS-SLM) using LCOS (Liquid Crystal On Silicon) is known. When a voltage is applied to a pixel electrode, liquid crystal molecules of the LCOS-spatial light modulator are rotated on a surface vertical to a substrate to change a phase modulation amount of an incident light. In this case, however, the phase modulation amount changes in a nonlinear manner relative to the voltage applied to the pixel electrode, and thus, there arises a problem that a desired phase modulation amount cannot be obtained.
A silicon substrate of the LCOS-spatial light modulator is processed through a semiconductor process, and thus, a thick silicon substrate cannot be obtained, and the mechanical strength is weak. As a result, as shown in FIG. 1, the silicon substrate is distorted due to stress generated at each process during an element fabrication. This decreases the flatness of the mirror surface of the LCOS-spatial light modulator. Further, due to the distortion of the silicon substrate, the thickness of a liquid crystal layer of the LCOS-spatial light modulator is uneven. This leads to the following problems: the phase modulation amount in each pixel differs depending on the thickness of the liquid crystal layer. Because of the variation of the phase modulation amount and the distortion of the reflective surface, a waveform surface reflected and output in the LCOS SLM is greatly distorted. Moreover, the phase modulation amount differs depending on each pixel. Specifically, a phase modulation amount φ (V, x, y) is expressed by the following equation, where x, y denote the position of the pixel, V denotes the voltage:Φ(V,x,y)=φ(V,x,y)+Φ0(x,y)[π rad]
By using this equation, the phase modulation amount φ(V, x, y) can be determined by a sum of the amount φ(V, x, y) dependent on voltage and the amount φ0(x, y) not dependent on voltage. It is noted that φ(V, x, y) is expressed by the following equation:φ(V)=2Δn(V)d(x,y)*2π/λ[rad]
Herein, Δn(V) denotes a birefringence relative to a polarization component having an electric field vibrating in a direction parallel to an orientation of liquid crystal. d(x, y) denotes the thickness of the liquid crystal layer at a position (x, y). In each pixel, a relationship between the voltage V and the φ(V, x, y) is nonlinear. Resulting from d(x, y), φ(V, x, y) takes a different value for each pixel. On the other hand, the φ0(x, y) arises mainly from the distortion on the reflective surface of the LCOS-spatial light modulator. Hereinafter, the nonlinearity between the voltage and the phase modulation amount and the variation in the phase modulation amount for each pixel, arising from φ(V, x, y), are collectively called a voltage-dependent phase modulation characteristic, and the variation in the phase modulation amount for each position (x, y), arising from φ0(x, y), is called a voltage non-dependent distortion. There are proposed methods for correcting the voltage-dependent phase modulation characteristic and the voltage non-dependent distortion (for example, Non-patent Documents 1 to 3).
There is also proposed a method of correcting a voltage-dependent phase modulation characteristic in a phase modulating module configured by a reflective light address spatial light modulator and a liquid crystal display, i.e., the phase modulation characteristic of the reflective light address spatial light modulator relative to the voltage applied to the liquid crystal display (for example, Non-patent Document 4). In the correcting method in the Non-patent Document 4, the voltage-dependent phase modulation characteristic is measured by using a polarimetric interferometer, a look-up table (LUT) is created for each block (one block corresponds to 4×4 pixels) based on the measurement result, and the voltage-dependent phase modulation characteristic is corrected by using the look-up table.
In another proposed method, a distortion of an output waveform surface of a reflective light address spatial light modulator is measured in a phase modulating module configured by the reflective light address spatial light modulator and a liquid crystal display, and the voltage non-dependent distortion is corrected by using a pattern cancelling the distortion (for example, Patent Document 1).
There is also proposed a reflective liquid crystal projector configured by a reflective light address spatial light modulator and a liquid crystal display device. In this device, an operation voltage to be applied to the liquid crystal display device is changed for each one of the blocks divided in a plurality present within a reading light irradiation surface of the reflective light address spatial light modulator, and thereby, an amount of writing light entering the reflective light address spatial light modulator is adjusted. Reading light obtained from a light source having a certain size will not become complete parallel light, and thus, an incident angle of the reading light differs depending on each block within the reading light irradiation surface of the optical writing spatial light modulator. However, the writing light amount is adjusted for each divided block, and thus, the output characteristic can be rendered uniform across all the blocks (for example, Patent Document 2).
Moreover, there is also proposed a method in which the reading light diagonally enters the reflective light address spatial light modulator, thereby achieving phase modulation of the reading light (for example, Non-patent Document 5).    Patent Document 1: WO 2003/036368    Patent Document 2: Japanese Patent No. 3071999    Non-patent Document 1: “Phase calibration of spatially non uniform spatial light modulator”, Applied Opt., “Vol. 43, No. 35, December 2004]    Non-patent Document 2: “Improving spatial light modulator performance through phase compensation”, Proc. SPIE, Vol. 5553, October 2004    Non-patent Document 3: “Active, LCOS based laser interferometer for microelements studies”, Opt. Express, Vol. 14, No. 21, October 2006    Non-patent Document 4: “Highly stable wave front control using a hybrid liquid-crystal spatial light modulator”, Proc. SPIE, Vol. 6306, August 2006    Non-patent Document 5: “Oblique-Incidence Characteristics of a Parallel-Aligned Nematic-Liquid-Crystal Spatial Light Modulator”, OPTICAL REVIEW, Vol. 12, No. 5 (2005)372-377    Non-patent Document 6: M. Takeda, H. Ina, and S. Kobayashi, “Fourier-transform method of fringe pattern analysis for computer-based topography and interferometry”, J. Opt. Soc. Am., Vol. 72, 156-160 (1982).